1. Field of the Invention
This invention relates to a polarization beam splitter used in lights of a plurality of wavelengths or bands, and relates, for example, to various optical apparatuses such as an image taking optical system, a projection optical system (projector), an image processing apparatus and a semiconductor manufacturing apparatus.
2. Description of Related Art
There is known a polarization beam splitter using dielectric material multi-layer film comprising two kinds of media. As shown in FIG. 12 of the accompanying drawings, it uses transmission (19) at Brewster's angle regarding P polarized light (18) incident on multi-layer film 17, and effects reflection (21) by the interference of the multi-layer film regarding S polarized light (20).
Generally, when the refractive index of the medium on the incidence side with an interface as the boundary is defined as n1 and the refractive index of the medium on the emergence side is defined as n2, the Brewster's angle θB is given by the following expression (1):tan θB=n2/n1  (1)
It is necessary that this relation is satisfied by a prism medium and a plurality of media forming the dielectric material. Among the refractive index np of the prism medium, the refractive index nH of a high refractive index layer forming the dielectric material and the refractive index nL of a low refractive index layer, it is necessary that the following relational expression is satisfied.
                              n          P                =                                                            n                H                2                            ⁢                              n                L                2                                                                    sin                2                            ⁢                                                θ                  B                                ⁡                                  (                                                            n                      H                      2                                        +                                          n                      L                      2                                                        )                                                                                        (        2        )            
Regarding the S polarized light, reflecting film by multi-layer film interference is constituted by the use of the reflectances of the medium H of the high refractive index layer and the medium L of the low refractive index layer. It is possible to realize reflecting film for the entire area of visible light by 20 to 40 layers. Regarding the S polarized light, by increasing the number of the layers of the film, it is possible to secure an angle characteristic and a wavelength characteristic widely.
On the other hand, as is disclosed in U.S. Pat. No. 5,042,925, there is known a polarization beam splitter sandwiching an adhesive agent having birefringence, instead of dielectric material multi-layer film, between prisms. This uses the refractive index difference of a birefringent material between a normal ray and an abnormal ray, and the refractive index difference is not great, but yet by using it at a great incidence angle of about 60°, one polarized light is selectively totally reflected to thereby realize polarized light separation.
For total reflection to occur, the incidence angle need be a critical angle θC or greater, and the critical angle θC is given by the following expression:sin θC=n2/n1  (3)
Also, as a polarization beam splitter using sub-wavelength structure (SWS) having a period of a used wavelength or less, there is known one as shown in FIG. 11 of the accompanying drawings wherein multi-layer film is etched into a rectangular grating shape. Here, the wavelength used is a wavelength supposed to be used in an optical element. When the supposed wavelength has a band, the wavelength band is defined as a used wavelength area, and the central wavelength thereof is defined as the used wavelength. For example, in the case of an optical element supposed to be used in visible light, the used wavelength is defined as the central wavelength of a visible area even if light incident on the element includes lights of the other wavelength areas than the visible area infrared to ultraviolet. High refractive index layers (H layers) 15 of TiO2 or the like and low refractive index layers (L layers) 16 of SiO2 or the like were alternately laminated, and they were etched so that the ratio of the pitch (period) of periodic structure to the medium (grating width), i.e., a filling factor f, might be about 0.5. The period of the rectangular grating forms structural birefringence as structure shorter than the wavelength.
In a rectangular grating as shown in FIG. 8A of the accompanying drawings wherein a medium 22 of a refractive index n1 and a medium 23 of a refractive index n2 (in this case, the air between gratings) are repeated at a ratio of a:b (i.e., the filling factor f=a/(a+b)), when a direction parallel to the grating is defined as TE and a direction orthogonal to the grating is defined as TM, it is known that the structural birefringence in the direction TE and the direction TM is represented by the following expressions (4) and (5), respectively:
                              TE          ⁢                                          ⁢                      n            TE                          =                                                            an                1                2                            +                              bn                2                2                                                    a              +              b                                                          (        4        )                                          TM          ⁢                                          ⁢                      n            TM                          =                                            a              +              b                                                      a                                  n                  1                  2                                            +                              b                                  n                  2                  2                                                                                        (        5        )            
At this time, irrespective of the ratio of a:b, nTE>nTM and therefore, in the direction TE, the refractive index difference between the H layer and the L layer is great, and in the direction TM, the refractive index difference between the H layer and the L layer is small. When an appropriate prism medium is adopted, the condition of the Brewster's angle is satisfied in the direction TM and the P polarized light can be transmitted. The thickness of each layer is independent on the condition of the Brewster's angle and therefore, by optimizing the film thicknesses of the H layer and the L layer, it is possible to form dielectric material multi-layer film. Thereby, the S polarized light is reflected and the function as a polarization beam splitter is obtained. This more heightens the degree of selection of a medium satisfying the condition of the Brewster's angle in P polarized light than a polarization beam splitter constituted by dielectric material thin film alone. Therefore, at the same time, it is possible to secure the reflectance in the S polarized light high. This leads to the feature that a polarization beam splitter covering the entire visible light area can be constituted by the order of 20 layers.